The present invention relates to a method for operating a combined-cycle power station. In particular, the invention relates to a method in which primary response and secondary response requirements are satisfied by a combined-cycle power station in a manner which protects the system, via the interaction of the thermal power from the fuel in the gas turbo group and an supplemental firing in the heat recovery steam generator.
With the liberalization of the energy markets, the competition is also becoming considerably more intense in the field of electricity supplies. This is leading on the one hand to rapid introduction of modern energy conversion methods, particularly those with very high efficiency. However, efforts are being made to look for more cost-effective options in the field of providing cold and hot reserves, as well. These requirements are satisfied to a major extent by gas turbo groups and combined-cycle power stations.
Gas turbo groups are able to provide comparatively very high power output gradients. This applies not only to cold starting but also to power output increases from any given power output point. Modern gas turbo groups are nowadays able to produce their rated power output from cold in 30 to 40 minutes.
As a result of the liberalization process, the electricity network operators are increasingly demanding primary response characteristics from the electricity generators"" power stations. The expression primary response relates to increasing the power output beyond the reported or currently produced actual power output of any given energy generator with a defined power output gradient. For example, the power output should be increased from the actual power output to satisfy a requirement for an approximately 10% greater power output within 10 seconds. This means that, in the event of a drop in the grid frequency (for example 0.5 Hz), the power stations must be able to produce an increase in the power output (for example 10% of the actual power) in a specific time (for example 10 seconds). It should then be possible to maintain this increase in the power output, in the sense of a secondary response, over a period, for example, of 30 minutes or more. The expression secondary response defines the maintenance of an additional power output above an actual power output, that is to say for example operation with an additional power output of, for example, 10% for a time period of, for example, 30 minutes.
The maximum power output gradient which can be achieved, the magnitude of the additional power output as a function of the actual power output currently being produced, and the maximum duration for providing the additional power output are thus of interest for questions relating to the primary response and secondary response. It should be possible to produce an additional power output, with the exception of the maximum power output, from any given load point. Increasing the power output above the rated power output is in this case subject to particularly stringent requirements.
The expression rated power output is in this case identical to the expression maximum continuous power output, that is to say a high power output for which the system is designed for continuous operation. The expression partial load in this case means a power output below the maximum continuous power output, while an overload means a power output above the maximum continuous power output. The term maximum power output will be used in the following text for the maximum output power which can be produced for a restricted time.
Increases in the power output are particularly critical during times at which there is a peak load on the grid system, during which the respective energy generators are already being operated at their maximum continuous power output (rated power output), and an unplanned event occurs at the same time, which necessitates a brief increase in the output power above the maximum continuous power output.
As already mentioned, gas turbo groups are able to produce comparatively very high power output gradients. For this reason, gas turbo groups are in principle suitable for primary response purposes.
However, nowadays, modern gas turbo groups are designed for efficiency reasons such that they operate in a wide upper power output range in the region of the maximum permissible design temperatures for continuous operation at the rated power output (upper process temperature), that is to say they are at their design temperature limit for continuous operation.
In this upper power output range, power output is controlled via the mass flow, by means of variable guide vanes at the compressor inlet. Increases in the power output in this range thus lead to the design temperatures being exceeded when the operating regime for normal operation is departed from, and this has a negative effect on the life, in particular of the components affected in the hot gas path.
However, this also means that any increase in the power output beyond the rated power output can be achieved only by supplying excess fuel to the gas turbo group, with the configuration that is nowadays normally used. This has the disadvantage that this method of operation as a departure from the normal regime results in the gas turbo group using up a very large number of equivalent operating hours (EOH=equivalent operating hours, OH=operating hours) and losing a very large number of operating life hours (for example 1.3 EOH/OH for steam injection or 1.5 EOH/OH for frequency response). This is particularly true when the overload has to be maintained for a long time.
A further critical situation can occur when the power output is increased rapidly with the power output subsequently being maintained through a secondary response time period during starting of the gas turbo group from cold. Particularly in the case of a cold start, the stabilization processes which are required for technical reasons at specific power output levels may make it necessary to provide holding points at which the power output is kept essentially constant. In the event of a sudden power output demand at the instant before or during such a holding point, it may become necessary to pass through these holding points, and to exceed the maximum permissible power output and temperature gradients, etc.
The invention is accordingly based on the object of providing a method for immediately, rapidly and temporarily increasing the power output of a combined-cycle power station.
In this context, the expression xe2x80x9cimmediatelyxe2x80x9d should be understood as meaning that the increase in the power output starts to become effective essentially without any delay after being demanded from the combined-cycle power station generators by the electricity marketing organization. xe2x80x9cRapidlyxe2x80x9d in this context should be understood as meaning that the power input is increased in a short time, that is to say that a high positive power output gradient can be produced. xe2x80x9cTemporarilyxe2x80x9d should in this case be understood as meaning that the additional power output is not decreased again immediately after being built up to the required maximum, but is kept essentially constant for a certain period of time before being reduced.
In this case, this relates to a combined-cycle power station having at least one gas turbo group, at least one heat recovery steam generator and at least one steam turbo group, with the gas turbo group comprising at least one compressor, at least one combustion chamber and at least one gas turbine. The heat recovery steam generator has at least one pressure stage, and the steam turbo group has at least one steam turbine.
In a combined-cycle power station such as this, air is compressed in a compressor, is then passed as combustion air to a combustion chamber, the hot gas which is produced there is passed to a gas turbine, and the exhaust gas from the gas turbine is used in a heat recovery steam generator to produce steam for a steam turbo group. The method is intended to make it possible to achieve rapid power output gradients as required on the grid system side, and to maintain the increased power output for a certain period of time.
This object is achieved in that an supplemental firing is provided for additional heating of the exhaust gas from the gas turbine, and in that the gas turbo group is supplied with more fuel in order to increase the power output, and the supplemental firing is switched on at the same time, for immediately, rapidly and temporarily increasing the power output of the combined-cycle power station, and in that the power output of the gas turbo group is reduced again to the extent that the additional steam power produced as a result of the supplemental firing is available via the steam turbo group as shaft power.
The essence of the invention is thus to use an supplemental firing to provide additional power. However, since this additional power is built up comparatively slowly due to the thermal inertia of the water/steam circuit, and is thus available only with a delay at the steam turbine end, or at the generator end if appropriate, the gas turbo group, which is suitable for such rapid increases in power output (primary response), is used first of all for the immediate and rapid rise in power output, and for the first phase of producing the increased power output. The additional supply of fuel to the gas turbo group can be reduced again to the extent that the additional power output which is provided by the supplemental firing and the steam turbo group also builds up in an effectively useful manner at the generator, thus protecting the gas turbo group. This means that the secondary response time period is essentially supported by the supplemental firing and by the steam turbo group.
A first embodiment of the method according to the invention is distinguished in that the temporary increase in the power output of the combined-cycle power station is provided solely via the supplemental firing and the steam turbo group, and in that the fuel supply to the gas turbo group is reduced to its original level again while the additional power output is being built up via the supplemental firing. This reduction in the output power from the gas turbo group to the original operating point as quickly as possible ensures that the gas turbo group is protected as much as possible, and is overloaded only to the minimum extent.
In a second embodiment of the method, the combined-cycle power station is a system in which the gas turbo group drives an electricity generator, and the combined-cycle power station has a steam turbo group with two or more steam turbines, in particular preferably with a high-pressure steam turbine and a medium-pressure or low-pressure steam turbine. The gas turbo group and the steam turbo group can likewise be arranged on one shaft, and the gas turbo group and the steam turbo group can drive an electricity generator via this common shaft (single shaft system). In this case, the generator may be arranged between the gas turbo group and the steam turbo group, and a coupling or clutch may be located between the steam turbo group and the generator.
In a further embodiment of the invention, the heat recovery steam generator and the steam turbo group are arranged in a closed water/steam circuit.
A further embodiment of the invention relates to an increase in the power output from the combined-cycle power station in the range from 5 to 15%, particularly preferably in the range from 5 to 10%, and this increase in the power output must in this case be built up in a time period of 5 to 30 seconds, particularly preferably in the range from 5 to 10 seconds. The additional power output must be maintained during a further time period in the range from at least 5 to 50 minutes, particularly during a time period of 15 to 30 minutes. The method according to the invention can be used efficiently particularly for power gradients such as these and in times with additional power outputs such as these, without in the process excessively overloading the components of the power station. The reduction in the power output from the gas turbo group to the original power output range can take place in a time period of 10 seconds to 5 minutes, in particular in the range from 30 seconds to 2 minutes.
In this case, the increase in the power output is frequently initiated or required as a result of a drop in the grid frequency in the order of magnitude of 0.1 to 3.0 Hz, particularly from 0.5 to 1.0 Hz.
In a further embodiment of the invention, the gas turbo group is already producing its rated power output before the increase in the power output, and the immediate, rapid and temporary increase in the power output of the gas turbo group is achieved by supplying excessive fuel to the gas turbo group. Particularly when the gas turbo group is being operated at its rated power output, there is a particular problem in increasing the power output. Since a gas turbo group which is being operated at its rated power output is also being operated at its maximum permissible upper temperature limit for continuous operation, any further increase in the power output can be achieved only by supplying excess fuel, which normally causes damage to the entire gas turbo group. Reducing the power output from the gas turbo group as quickly as possible after it has provided the primary response power output is therefore of particular interest in this situation.
In a further preferred embodiment of the proposed method, the supplemental firing is arranged upstream of the heat recovery steam generator and/or within the heat recovery steam generator in the flow direction of the exhaust gas from the gas turbo group. The supplemental firing may likewise be arranged outside the exhaust gas flow from the gas turbo group, and may have a fresh air supply, and the burnt gas from the supplemental firing is mixed with the exhaust gas from the gas turbo group. In this case, the exhaust gas from the gas turbo group can be mixed with the burnt gas from the supplemental firing upstream of the heat recovery steam generator and/or within the heat recovery steam generator in the flow direction of the exhaust gas from the gas turbo group.
Further preferred embodiments of the invention are described in the dependent claims.